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"SS Blackening in Dairy Industry"


Dear All

In dairies a normal cleaning operation includes caustic soda (1.5-2%) and nitric acid (1%). The solutions are circulated over SS 304/ 316 surfaces at temp not less than 70oC. If the iron content of the water is pretty high we observe over a period the heat treated surfaces are changing colours, first light yellow - yellow - brown and finally black. Can any body please help what are this depositions and how to remove this?

Malay Kumar Saha
ood Technologist - Kolkata, West Bengal, India

First of two simultaneous responses -- 2005

You could test the surface with ESCA or XPS, but it sounds like you have iron oxide forming on the surface and are seeing the basic colors of iron oxides. Iron forms ferric oxide (yellow to orange to brown) and magnetite (gray to black, with less oxygen). These can be removed with phosphoric acid or citric and organic acid combinations.

Daryl L. Roll
Costa Mesa, California, USA

Second of two simultaneous responses -- 2005

Is the black material biologic in nature? Slime molds are common in the food processing industry.

tom pullizzi animated    tomPullizziSignature
Tom Pullizzi
Falls Township, Pennsylvania


Thanks a lot for your responses.
Now answering your questions.
This is not a biological one.
I am also convinced that this is iron oxide but it is very difficult to remove with phosphoric acid. What about Na-Gluconate or potassium permanganate?

I have tried 1.5%-2% H3PO4

results can't be viewed.


Malay Kumar Saha
- West Bengal


If you have the capability to heat to 70 °C then I would suggest that you use Citric Acid at about 50 grams / liter and maintain circulation for several hours, it will clean the Stainless Steel .

John Tenison-Woods
John Tenison - Woods
- Victoria Australia


You could use soft water / demineralised water for cleaning. Corrosion might reduce over a few months of cleaning.

Ashish Shah
Yogurt - Bangalore, Karnataka, India

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